This invention relates to systems and devices for preventing fires, and for minimizing fire danger, from overheating and overloading conditions that might arise in or affect electrical wiring in residential and commercial installations.
Many residential and commercial fires result from causes that are generally, and sometimes inaccurately, described as faulty electrical wiring, or electrical circuit failures. One common cause of such failures is overheating of or by an extension or other power cord because of circuit overload or other conditions, separately or in combination. For example, when a temperature is reached within or outside the extension cord at which the synthetic or rubber compound insulation melts or decomposes, a heated interior wire may become exposed and come in contact with and ignite flammable material. Also, insulation failures can also allow high amperage arcing to occur between interior wires. The result is a type of fire which is known to cause loss of a number of lives and much residential and commercial structure damage each year. Electrical circuits and interconnecting cords and cables are designed to function with an expected degree of resistive heating, and to accommodate temperature buildup as heat is conducted along the circuits from one point to another. A long extension cord, for example, transfers heat along its conductive wiring from the outlet or source from which it derives current, and from an appliance or device at which electrical power is consumed. Combinations of operating factors thus can result in overheating of an extension cord, for example, and temperatures can reach a level at which decomposition begins and incipient danger exists.
Electrical codes require protection against electrical overload conditions in permanent installations by compelling inclusion of fuses or circuit breakers in the wiring system between the main power supply and the points of usage in a residential or commercial building. There are, nonetheless, a number of types of potential wiring originated fires that are not precluded by fuse or circuit breaker protection, including those mentioned above.
Although product manufacturers and electrical equipment code requirements provide instructions as to preferred and limiting conditions for use of components such as receptacles, extension cords, and circuit breakers, there can be no guarantee that users will comply with these stated instructions. Extension cords, for example, are nominally intended to carry from 1200-1600 Watts, but users may in practice drastically overload an extension cord in a number of ways. For example, there is a tendency to use extension cords and circuits beyond their rated capacities, as by attaching a cord to an appliance of higher power than is nominally permitted. At the receptacles and outlets, there is a tendency to attach multiple devices, and if too many are coupled in this, may exceed the load carrying capacity of the receptacle even though individual appliances and devices may not demand much power individually.
Special problems can arise from environmental and building conditions. For example, when exterior temperatures in a region approach or exceed 100xc2x0 F., the temperature in attics, under-roof crawl spaces, and inside walls can approach considerably higher levels. In these uninsulated interior spaces the temperature can: be 40 to 60xc2x0 F. higher than the outside air temperature from the sun heating the exterior surfaces, which transmit radiant energy to heat the interior spaces. Extensions and other wiring are often placed in such spaces. The insulation of such wiring can approach threshold levels merely because of extremely high ambient temperature, because of being in a confined space, or because of an increase in resistance heating caused by increased ambient temperature. The resistance of copper conductors increases by a factor of 0.0047 for each degree Celsius above zero, thus increasing resistive heating with temperature when current is constant. The effect can be compounded if multiple wires are run in the same space or, as in the case of extension cords, the wire is bundled or coiled to fit within a limited space. Under these circumstances, relatively minor overheating of lines, receptacles and/or appliances can substantially increase danger of combustion.
Extension cords or cord sets, power cords, and other electrical power distribution circuits come in a variety of configurations. One very widely used type of cord employs a type of polyvinyl chloride (PVC) insulation that permits safe operation only over a temperature range of +32 to +140xc2x0 F. This widely used type of cord is at serious risk of experiencing excessive temperature due to the types of overload and ambient temperature factors discussed above. Due to the low maximum safe operating temperature of only 140xc2x0 F., the user often fails to appreciate the risks involved. While other types of insulation can safely withstand wider temperature ranges, e.g., another type of insulation rated for the xe2x88x9258xc2x0 to +221xc2x0 F. temperature range is readily available, such insulation causes the cord to be significantly more expensive. Consumers apparently consider the increased expense of the higher temperature range insulation to be unjustified by the corresponding increased safety because the lower temperature rated cording enjoys tremendous commercial success. Accordingly, a need exists for an inexpensive way to improve the safety of electrical power cording, and especially the type of electrical power cording that uses insulation having a low safe operating range.
Electrical fuses protect substantially only against current overloading, as do circuit breakers. Furthermore, circuit breakers can malfunction and fail to operate. Under such conditions, they may overheat, and even if they do not themselves fail, they act as a heat source for interconnected elements in the wiring system. The larger the gauge of the copper wires used in an extension cord, for example, the more current it can carry for each degree of temperature rise. At the same time, the copper, which is an extremely good thermal conductor, becomes more efficient in transferring heat energy along its length. Consequently, there can be bi-directional interaction between different points in an electrical system, so that a fire need not necessarily arise at some point of malfunction, but may be initiated at a remote location that is in effect a weak link in the system. It is evident that protection is needed whether there is an overheating danger from causes other than electrical malfunction, or some misoperation or misuse of electrical equipment. It is obvious, furthermore, given the large installed inventory of receptacles, outlets, circuit breakers and fuses, as well as the widespread employment of semipermanent adapter outlets and extension cords, that the patterns of usage in residential and commercial buildings will not substantially change. Consequently, the dangers inherent in many usage habits will remain unless a convenient means is found for protecting against fire danger from these causes. For these and other reasons, there is a need for compact and inexpensive safety elements which cooperate with standard electrical circuitry to protect against the types of failures in electrical wiring systems that endanger individuals and cause damage or destruction to buildings.
A few extension cords, power cords, and the like have incorporated inexpensive fuses in an attempt to improve safety at a minimal increase in expense. However, such attempts have generally been unsuccessful. Typical fuses are susceptible to nuisance tripping in response to momentary overcurrent conditions, such as normally occur when starting a motor. Such situations are considered nuisance tripping because the momentary overcurrent conditions pose no fire hazard. Moreover, traditional fuses are current, not temperature, limiting devices that fail to account for ambient temperature conditions. An amount of electrical current that is safe in a 75xc2x0 F. ambient temperature may very well be unsafe in a 130xc2x0 F. ambient temperature. Accordingly, a fuse which permits safe operation at a low ambient temperature is likely to allow unsafe operation at a higher ambient temperature. Alternatively, a fuse which prohibits unsafe operation when a cord operates in a higher ambient temperature is likely to experience nuisance tripping when the cord operates at lower ambient temperatures because an amount of current that is unsafe at a higher temperature may be perfectly safe at a lower temperature.
Alternatively, a fuse which prohibits unsafe operation when a cord operates in a higher ambient temperature is likely to experience nuisance tripping when the cord operates at lower ambient temperatures because an amount of current that is unsafe at a higher temperature may be perfectly safe at a lower temperature.
Time delay fuses are also available for incorporation in series with extension cords, power cords and the like. Time delay fuses could potentially limit the nuisance tripping caused by momentary overcurrent, such as motor starting. However, conventional time delay fuses are large, complex structures that would for many applications, if integrated with electrical power cording, raise the price even more than the price increase necessitated by the use of higher temperature insulation. Moreover, time delay fuses are current limiting devices that fail to account for the ambient temperature in which the extension cord, power cord, or the like is operating. Conventional time delay fuses, when connected in series with an extension cord, power cord, or the like, either experience serious nuisance tripping while adequately protecting against excessive temperature or fail to adequately protect against excessive temperature.
Systems and devices in accordance with the invention provide a heat responsive function to open a circuit when an overheating condition exists that may be wholly independent of electrical overload. They may utilize a circuit configuration in which a part of a spring-loaded element that conducts current in a circuit is physically retained in position in the circuit by a thermally responsive alloy or other joinder element. The joinder element is in a thermally conductive path with the circuit or device, and any surrounding environment, that has the potential to cause overheating. A protective device of this kind responds to some physically separate overheating condition, regardless of the source, by causing the heated joinder element to yield at a predetermined threshold, so as to release the spring-loaded circuit element and open the circuit. The threshold temperature is selected in accordance with the operating parameters of the unit of concern, such as in relation to the temperature at which the insulation of an extension cord begins to decompose or degrade. Alternatively, the threshold temperature may be chosen with respect to the permissible temperature limits for a circuit breaker or an appliance. In any event, whatever the cause, whether it is due to an environmental temperature condition that exposes bare wire in an extension cord, or an electrical malfunction, or overloading of the electrical circuits themselves, the current is cut off and the power using element and associated circuits no longer function as heat sources.
Such protective devices in accordance with the invention can be fabricated as integral parts of complete units, or they can be fabricated as separate adapters to be attached to existing installations. By this means, an adapter can be interposed between an extension cord and a wall receptacle, between an appliance and a cord, or between a wall receptacle and a multi-outlet plug to assure that the circuits do not overload or overheat due to internal or external causes.
In a more particular example of a device in accordance with the invention, a male plug for an extension or appliance cord may either be integral with a cord or in the form of a separate adapter unit. The male plug is fabricated with extending prongs at least one of which includes an interior conductive spring element and is in electrical circuit with the conductor or unit to be protected. The spring element is configured to have conductivity characteristics so that its resistive heating does not exceed that of the conductors to which it is coupled. The spring element is held under tension or compression within the completed circuit by the temperature responsive joinder material. In most instances, the temperature responsive material is a bismuth and lead-containing alloy having a melt or yield temperature in the range of 130 to 350xc2x0 F., usually less than 150xc2x0 F. when used in conjunction with electrical power cording having insulation with a maximum safe temperature rating of around 140xc2x0 F. The temperature that is needed for actuation in particular circumstances can be quite precisely varied however, by selection of the alloy constituents.
When the device takes the form of an adapter, it includes female terminals on one side into which the male connectors of a conventional extension or appliance cord may be inserted. The adapter plug then can be inserted in a wall receptacle or other outlet, where it is responsive to above threshold temperature levels, whether caused by overheating from the extension cord/appliance side or from the receptacle or outlet itself, or from the circuit breaker side. Consequently the thermal protection device functions in response to sources from at least two directions, and irrespective of the manner in which overheating arises. Power to an insulated cord or associated appliance is shut off, eliminating power drain that may alternatively affect the wall receptacle or the circuit breaker. In addition, the protective device functions as a secondary current overload protector, responding, for example, to a current load if a circuit breaker fails in the closed circuit condition. When the male plug is integral with an extension cord, it preferably includes stress relief elements to resist the strain of pulling on the cord to detach the plug. All devices are of materials consistent with standard building codes and industrial design practices.
The thermal protective element can incorporate a means for indicating that the threshold condition has been exceeded and the device is in the open condition. As one example, a transparent window in at least one side of the element enables viewing of the position of one or more of the spring-loaded elements. As another example, an element moveable in the side of the protective device can be engaged and moved outwardly by the spring loaded element when the fusible material yields, indicating the failure condition in a manner visible from the outside.
In another example in accordance with the invention, existing circuit breakers can be augmented by an augmentation or backup protective device that is responsive to both thermal and electrical loads. A small unit having a mechanically biased conductive element held in place by a chosen temperature responsive alloy is electrically coupled into the circuit breaker and receives power-carrying wires for supplying exterior circuits in the building. The unit includes means for exerting pressure on the wire for better electrical contact, but limits conduction of heat to the exterior by incorporating a thermally non-conductive element in the force-exerting structure. The augmentation device is designed to have a maximum current limit greater than the circuit breaker, so that as long as the circuit breaker is functional, the augmentation device is not actuated by a current overload. However, in the event of circuit breaker overheating, or failure for any other reason, the augmentation device provides an added protection, including safeguarding against electrical overload if permitted by the circuit breaker failure condition.
In accordance with another feature of the invention, a separate circuit unit is provided that can be incorporated wherever insulated cable is disposed in the likely path of a fire. The interior-circuit device includes parallel conductors for each of the three lines of a grounded cable, one of the conductors including a conductive spring held by a meltable alloy in circuit with a current carrying wire, such as the positive line in a building system. This in-circuit device opens under any of several dangerous conditions, including excessively high wire temperature, excessively high environmental temperature, and severe current overload.
A different circuit unit in accordance with the invention is also provided that couples directly into the circuits of appliances or other electrical devices and is responsive to appliance temperature. By actuating when the appliance overheats, this unit independently assures against excessive heating of the appliance if the conventional temperature regulating element, usually a bi-metallic switching device, fails. The unit may be compact, and replaceably inserted into a small recess in the appliance.